As compared to alumina, the commercially predominant electronic ceramic, aluminum nitride ceramics potentially possess superior characteristics for electronic packaging applications with respect to electrical insulation, high thermal conductivity (above 120 W/m-K), thermal expansion match to silicon devices, and low dielectric constant. Aluminum nitride substrates are potentially useful where high heat dissipation is required in a microelectronic package, such as in a multilayer metal-ceramic package for high power devices. Aluminum nitride ceramics for microelectronic applications must therefore be capable of accommodating metallized components, polymeric layers and heat generating, high power electronic devices.
Prepared from aluminum nitride powders, in order to achieve suitable properties the ceramic must achieve a certain density, at least about 90%, preferably greater than or equal to about 95%, of theoretical. Aluminum nitride with no sintering additives decomposes below the temperature required to sinter it to maximum density. However, densification can be achieved at lower temperatures by the use of sintering aids.
Sintering aids liquify at temperatures below the decomposition and pure compound sintering temperatures for the ceramic, and promote densification of the ceramic grains by i) a particle rearrangement process mediated by capillary forces between the wetting liquid and the solid particles, and thereafter, ii) a dissolution and precipitation process. In this process, solid is preferentially dissolved at regions of high curvature (small particles) and redeposited at regions of low curvature (large particles). In addition, solid is preferentially dissolved at regions of solid-solid contact and redeposited away from the contact areas. At the later stages of the liquid sintering cycle, microstructure is refined via grain growth and coalescence processes.
Different combinations of sintering aids provide various compounds in situ which melt at different temperatures. The temperatures at which sintering occurs has an effect on the progress of the different types of sintering processes, and thus the microstructure and the final properties of the sintered ceramic body. Sintering aids also function to increase thermal conductivity of the sintered aluminum nitride body by gettering oxygen from the aluminum nitride powder. Thus, an effective sintering additive must form a liquid at low temperature capable of dissolving and reprecipitating aluminum nitride without oxidation of the aluminum nitride. Not every liquid at sintering temperature will be able to getter oxygen and densify the ceramic.
All commercially available aluminum nitride powders contain oxygen as an impurity. This oxygen primarily takes two forms in the powder, as an alumina coating on each of the powder particles, and as dissolved oxygen impurity within the crystalline lattice of the aluminum nitride particles. A minor amount will be tied up as an oxide of any metal impurities which may be present. At a given sintering temperature, only a certain amount of oxygen, primarily from surface alumina and secondarily from other sources, will be available for reaction (hereinafter "available oxygen").
Upon densification, the volume of the green body, and for multilayer structures the volume of the metal lamina contained in the green body, together with the linear dimensions of the body, decrease as a function of both the temperature experienced and the particular material involved. If the metal and ceramic shrink at different times and rates, this shrinkage mismatch leads to residual stresses between the different constituent materials in the sintered body and distorts the final shape of the body. In order to maintain the exacting geometric tolerances required by the electronic packaging industry for multilayer ceramic based packages, it is necessary that the ceramic and the metal sinter at approximately the same rate.
Thus it is desirable to facilitate efficient sintering of aluminum nitride at particularly low temperatures to mediate the problems associated with different sintering rates and thermal expansion mismatches between the ceramic and metal portions of a multilayer electronic package.
The use of lower sintering temperatures by the art, however, has generally resulted in properties degrading from the desired theoretical levels. For instance, the electrical resistivities of some aluminum nitride sintered bodies, particularly those AlN substrates sintered with calcia and yttria containing sintering aids as discussed below, have been found to be unacceptably low (on the order of 10.sup.8 .OMEGA.-cm) for certain electronic packaging applications, and therefore, the substrates are not believed suitable for use as insulating substrates for electronic packaging applications.
At least some of these less than desirable properties may result from the failure of the sintering aids to either form an effective sintering liquid needed to densify the ceramic or to remove dissolved oxygen from the AlN lattice, and/or from the formation of an additional phase or additional phases within the AlN structure which comprise reaction products of the sintering aid(s), aluminum and oxygen.
Sintering aids for AlN which have been disclosed in the art include Group IIa, Group IIIa, and/or rare earth compounds, including calcia and yttria, among others. Resulting AlN sintered bodies are disclosed to contain alkaline earth-aluminates, Group IIIa-aluminates, rare earth-aluminates, and/or AlON.
U.S. Pat. No. 4,618,592 discloses the use of sintering aids for aluminum nitride which are at least one metal element selected from alkaline earth metals, lanthanum group metals and yttrium or a compound thereof.
U.S. Pat. No. 4,746,637 discloses sintering aluminum nitride powder in mixture with a rare earth compound and an alkaline earth metal compound. U.S. Pat. No. 5,077,245 discloses sintering aluminum nitride using as sintering aids at least one metal or compound of a Group IIa metal such as Ca and at least one metal or compound of a Group IIIa metal such as Y and rare earth compounds. Mixed oxides of Group IIa/IIIa metals and alumina were identified in aluminum nitride sintered with these sintered aids. In Sainz De Baranda, Pedro, "The Effect of Calcia and Silica on the Thermal Conductivity of Aluminum Nitride Ceramics", A doctoral dissertation, Rutgers University, (Vol. 52/07-B of Dissertation Abstracts International, p 3846.), two ternary oxide second phase compounds were identified in aluminum nitride bodies sintered using yttria and calcia (calcium nitrate) sintering aids, namely CaYAlO.sub.4 and CaYAl.sub.3 O.sub.7. Sainz De Baranda also observed that with the addition of calcia as a sintering aid, the second phase became more wetting of the grain boundaries, and low dihedral angles were observed (Pages 193 and 199-200).
U.S. Pat. No. 5,165,983 discloses a method to sinter a plurality of AlN plates containing oxides of aluminum, rare earth, and Group IIIa metal elements superposed on a ceramic support base with a ceramic powder interposed between the base and the plate and between the plates.
Japanese Kokai J02-275,769 discloses additions of aluminum, calcia and boria to aluminum nitride powder, followed by sintering at 1400-2000 degrees Centigrade. However, to achieve a fully dense body having a thermal conductivity of 192 W/m-K, the compositions were sintered at 1800 degrees Centigrade for 4 hours.
Japanese Kokai J62-176,961 discloses additions of alumina, calcia and boria (as well as others) to aluminum nitride to achieve a sintered body with improved density and thermal conductivity. Boria, however, melts at about 450 degrees Centigrade which presents difficulties in electronic packaging applications. For example, it is necessary to remove substantially all residual carbon from substrates that are used in electronic applications. The low melting boria hinders this so-called binder burn-off process.
Japanese Kokai J03-218,977 discloses the addition of 0.1-10 weight percent of a glass powder sintering aid to the aluminum nitride powder prior to sintering. The glass powder consists of 0-38 mole % alumina, 30-80 mole % boria and 20-56 mole % calcia.
In weight percent, it is 0-28 weight % alumina, 27-77 weight % boria and 23-64 weight % calcia. The aluminum nitride body is sintered at a temperature greater than 1650 degrees Centigrade which is undesirably high. The resulting aluminum nitride samples have a maximum thermal conductivity of 110 W/m-K which, while better than alumina, is considerably less than pure aluminum nitride. Further, the majority of samples had a thermal conductivity of 100 W/m-K or less.
Beyond this art, we have now found that particular second phase compositions retained in the sintered aluminum nitride ceramic body following sintering, while effective in providing density to the aluminum nitride during the sintering process and enhanced thermal conductivity in the sintered body, contribute to the degradation of other necessary characteristics in the electronic packaging substrate, particularly electrical resistivity.
We have found that in sintered aluminum nitride bodies, the presence of residual calcium-aluminate (calcia) containing species is associated with very low resistivity. In fact, when certain calcium aluminate species are exposed during sintering to the environment of a refractory metal furnace, the second phase becomes conductive. This is a deleterious characteristic for an electronic packaging substrate, which must be insulating in order to isolate conductive paths for carrying signals and power to and from semiconductor chips.
Although sintering for longer times and/or at higher temperatures can volatilize or decompose calcia-based species from monolithic ceramics, this tends to degrade other desirable characteristics, as noted above. Also, for co-fired, heavily metallized multilayer ceramic packages, this technique is not effective. In the metal-ceramic laminates, dense metal planes sinter very early during the sintering process, and trap volatile species in the ceramic layers between them. Extended or higher temperature sintering are not able to remove these species from the ceramic.
Second phase compositions which remain trapped in the ceramic layers between the dense metal layers in co-fired multilayer substrates exhibit very low resistivity and result in sintered body microstructures which are unsuitable for electronic packaging applications.
Thus, it is an object of the present invention to produce an aluminum nitride body that has high electrical resistivity, is fully dense and is highly thermally conductive by sintering at a lower sintering temperature than has heretofore been feasible and which will allow the production of the aluminum nitride body at a reduced cost.
It is another object of the present invention to produce an aluminum nitride body that has high electrical resistivity, is fully dense and is highly thermally conductive, by sintering at a temperature which is compatible with metal-ceramic laminate processing.
It is still another object of the present invention to produce an aluminum nitride body that has high electrical resistivity, is fully dense and is highly thermally conductive, by sintering at low sintering temperatures, while being able to control the certain compositional and microstructural characteristics of the ceramic body so as to provide substrates which are suitable for electronic packaging applications.
These and other purposes of the present invention will become more apparent after referring to the following detailed description of the invention.